I'd say there's a fourth reason, which is that "Joe Sixpack" has a dvd player and is wise to dvd release patterns. I mean, in my workplace, for instance, I hear this same conversation any time the subject of going to the movies comes up: "Considering what Hollywood is releasing nowadays, is it really worth paying out the ass to see something that I can watch in more comfort and for less money at home in three months? HELL NO!!!" Every single time. It's just getting harder and harder to pry people out of their homes for a theatrical movie.
It's not like Serenity was the only movie that underperformed this week - Flightplan, despite being #1 again isn't exactly burning up the charts. A couple of years ago a movie with a solid budget might have been guaranteed at least $20 million at opening but that's not the case anymore and I think the saturation of the dvd market has a lot to do with it.
Very true. It makes me wonder why anything less than a tentpole film is given a theatrical release anymore. Eveybody nowadays is about convenience and comfort at lower prices. And I can understand it. The bitter irony of course is that the films this mentality ends up hurting are those like Serenity that offer real reason to get out of the home and into the theater.